THE OLFACTORY NERVES. 425 



many facts. Animals in which these nerves have been di- Functionofthe 

 vided are no longer affected by odors of any kind, and, gen- first pair of 

 erally speaking, the greater the development of these nerves, nerves - 

 the acuter is the sense of smell. In persons in whom this sense has 

 been defective or totally absent, or in those who have been troubled with 

 unpleasant odors of a subjective kind, post-mortem examinations have 

 shown a corresponding absence or lesion of these nerves. 



In man, the proper olfactory organ is formed by the distribution of the 

 olfactory, or first' pair of nerves, on the mucous membrane which covers 

 the upper part of the nose, the internal set of filaments being disposed 

 on that of the septum, the external on that of the superior and middle 

 spongy bones. The membrane is very vascular, and covered with a thick, 

 pulpy epithelium. The filaments distributed to it have lost the white 

 substance of Schwann. It is those parts alone to which these filaments 

 are distributed which possess the sense of smell, the adjacent cavities, 

 as, for example, the frontal sinuses, not participating in the function, as 

 has been proved by the injecting of the vapor of camphor or other odo- 

 riferous bodies into them. It seems to be necessary for the vaporous or 

 gaseous substances to be dissolved in the moisture which covers the ol- 

 factory membrane in order to their exerting a proper effect. If, by 

 chance, the membrane is too dry, the sense of smell is temporarily lost, 

 and the same likewise occurs if it be unusually moist. 



From the mode of distribution of the olfactory nerve, it follows that* 

 the sense of smelling is restricted to the upper portion of Limited re ion 

 the nasal cavity ; and, for this reason, when we desire to de- for the sense of 

 tect odors with unusual precision, the air is drawn violently smell> 

 into that region by sniffing. On the contrary, we avoid the perception 

 of odors by breathing through the mouth, or, as the common Conditions for 

 phrase is, by holding the nose. Since the perfection of the its perfect ac- 

 sense requires that the olfactory surface shall neither be too i 

 dry nor too cold, an advantage is gained by placing it high in the cav- 

 ity, where it is free from the disturbing effects of the dry air introduced 

 by inspiration, which becomes moistened and warm before it reaches the 

 place of action. 



Just as we make a distinction between a musical sound and a noise, 

 so should we distinguish between an odor and such impres- Distinction be _ 

 sions as arise from tickling, pressures, the use of snuff, mus- tween odors 

 tard, pepper, and pungent bodies, for these act as mere irri- 

 tants, and many of them can produce analogous effects on other portions 

 of the surface of the skin. Odors do not give rise to the impressions of 

 pain, and, indeed, the nervous mechanism having charge of the action is 

 totally different in the two cases. Odors operate, as we have said, upon 

 the olfactory nerve, but these other impressions are made upon the nasal 



